1. Field of the Invention
The present invention relates to an optical system for imaging pickup, and more particularly to the optical system for imaging pickup to shorten total length and improve image quality for applying to electronic products.
2. Description of the Related Art
In compact electronic products such as digital still cameras or mobile phone cameras, an optical system for imaging pickup is generally installed for capturing images of an object, and the optical system for imaging pickup tends to be developed with a compact design and a low cost, while meeting the user requirements of good aberration correction ability, high resolution, and high image quality.
In general, a conventional optical system for imaging pickup of a compact electronic product comes with different designs, including the two-lens, three-lens, four-lens, and five-or-more lens designs. However, if the imaging quality is taken into consideration, the optical system for imaging pickup with the four-lens or five-lens designs has advantages on image aberration and modulation transfer function (MTF) performance, wherein the five-lens design having a higher resolution than the four-lens and five-lens designs thus being applicable for electronic products requiring the high quality and high pixels.
In various compact designs of the five-lens optical system for imaging pickup having a fixed focal length, the prior art adopts different combinations of positive and negative refractive powers as disclosed in U.S. Pat. No. 7,480,105, adopting the first lens element with negative refractive power and the second lens element with positive refractive power are used. Due to the two cemented glass lens elements, the manufacturing cost is higher which is unfavorable to the cost control in mass production. In addition, the glass lens element generally is thicker than the plastic lens element, and it is difficult to make an aspheric surface of the glass lens element and reduce the total length of the optical system. If the first lens element with negative refractive power is used, it will be more difficult to reduce the total length of the optical system.
In products such as compact digital cameras, web cameras, and mobile phone cameras, the optical system for imaging pickup requires a compact design, a short focal length, and a good aberration correction. As disclosed in U.S. Pat. No. 7,502,181, the fourth lens element with negative refractive power and the fifth lens element with negative refractive power are adopted to increase the back focal length and the total length of the optical imaging system, so that it is difficult to shorten the total length of the optical imaging system. In various five-lens designs of the optical system for imaging pickup with a fixed focal length, the first lens element has a convex image-side surface to facilitate a good balance between an increase of the view angle of the optical imaging system and the effect of the aberration correction, or the fourth lens element and the fifth lens element with different refractive powers are adopted, and the fourth lens element or the fifth lens element has an inflection point, but such optical system for imaging pickup fails to meet the requirements for a good aberration and a reduced total length of the optical imaging system. The present invention provides a more practical design for shortening the optical imaging system, adopting a combination of refractive powers of five lens elements, convex and concave optical surfaces. Wherein the fourth and fifth lens elements have positive refractive power and negative refractive power respectively, and this complementary combination with a telecentric effect is favorable for reducing the back focal length and the total length of the optical system for imaging pickup effectively as well as further improving the image quality and applying the optical imaging lens assembly to compact electronic products.